Generally, so called squirrel cage electrical induction machines are constructed with a laminated rotor core, copper rotor bars which extend through that core and copper or copper alloy end rings. FIG. 1 illustrates one conventional construction of a squirrel cage electrical induction machine. Thus, rotor bars 1 are located and fitted within slots 2 in a rotor core 3. These rotor bars 1 are usually rectangular or round drawn copper bars. The rotor bars 1 are connected to an end ring 4 that acts to short circuit the rotor bars 1 at each end of the rotor core 3. As can be seen, the rotor 3 is secured about a shaft 5, which in operation rotates.
In high speed applications, the end rings 4 are generally made from high strength copper alloy or copper chrome. Furthermore, in addition to using high strength copper alloy, these end rings 4 may be further reinforced by external banding in the form of carbon fibre over-wraps 6.
There are several technical difficulties encountered in designing high speed induction machine rotors using this approach:                a) The maximum strength of the available copper alloys (up to 250 MPa) is substantially less than that of the available lamination materials (up to 700 MPa).        b) The rotor bars 1 are constrained both by their fitment within slots 2 in the rotor core 3 and by their attachment to the end rings 4. Hence there is a potential (significant) problem with differential radial growth (thermal and centrifugal) between the end rings and the core.        c) Any banding system has to be pre-tensioned to eliminate problems due to centrifugal growth and loss of dynamic balance.        d) The thermal expansion coefficients of the steel laminations and the copper end ring material are mismatched by a factor of at least 2. The thermal expansion coefficient of carbon fibre (if used as a banding) is substantially less than for both steel and copper.In view of the problems with this conventional high speed induction machine construction, an alternative has been proposed generally illustrated in FIG. 2. This approach is described in U.S. Pat. No. 5,512,792 and European Patent No. 0609645. Referring to FIG. 2, it will be seen that radial laminations in the form of a rotor core 23 are presented between end rings 24 with rotor bars 21 extending in slots 22 between these end rings 24. The core 23 and end rings 24 are clamped together using steel end plates and tie bars which pass through both the laminations of the core 23 and the end rings 24. It will be noted that there is no through shaft passing through the centre of the rotor core 23 in comparison with shaft 5 in FIG. 1. Instead of such a shaft, the end plates 25 are shaped with integrally formed and forged stub shafts. The end rings 24 in such circumstances have previously comprised copper alloy discs. These discs are usually machined from copper plate. The maximum stress in the end rings 24 is limited by the fact that the end rings do not have a central hole for a shaft upon which the core 23 is rotated. In such circumstances, in the absence of special provision in the rotor clamping system, the end rings 24 are subject to thermal expansion, which is about twice that of the steel laminations of the core 23. This significant differential thermal growth may therefore be present in the end regions and hence there is a potential for breakage of the constrained rotor bars 21. Furthermore, long term stability and dynamic balance of the rotor 23 relies upon the continued clamping pressure within the rotor core 23. Thus, this clamping pressure must be maintained throughout the rotor life at all times and at all rotor speeds. It will be appreciated that this is difficult to achieve.        